The present invention relates to a frame-supported pellicle used for dustproof protection of a pattern-bearing photomask used in the photolithographic patterning works of, for example, a resist layer in the manufacturing process of various kinds of electronic devices such as semiconductor devices, liquid crystal display panels and the like. More particularly, the invention relates-to a frame-supported pellicle which is easily demountable from the photomask on which it is mounted so as to ensure good replaceability of pellicles in the patterning works.
As is known, the photolithographic method is well established in the patterning works for the manufacture of semiconductor devices, such as LSIs, VLSIs and the like, liquid crystal display panels and other fine electronic materials, in which a photoresist layer formed on the surface of a substrate such as a semiconductor silicon wafer is pattern-wise exposed to ultraviolet light through a pattern-bearing transparency called a photomask followed by development of the latent images to form a patterned resist layer. In view of the extremely high fineness and precision required in this patterning work, it is very important that the photomask is absolutely dust-free since, when dust particles are deposited on the photomask, the ultraviolet light passing through the photomask is necessarily scattered by the dust particles to give a great adverse influences on the quality of the reproduced pattern such as fidelity to the photomask pattern and contrast of the reproduced images.
It is therefore a usual practice that the photolithographic patterning work is conducted in a clean room under an atmosphere of air freed from any floating dust particles as completely as possible although a perfect dust-free condition can hardly be obtained even in a clean room of the highest class. Accordingly, it is also a usual practice that deposition of dust particles from the atmosphere on to the photomask is prevented by mounting a flame-supported dustproof pellicle on the photomask. The frame-supported dustproof pellicle mentioned above is an integral device consisting of a frame made from a rigid material, such as an aluminum alloy, and having parallel end surfaces and a thin, highly transparent film of a polymeric resin, which is called a pellicle membrane, spread over and adhesively bonded to one end surface of the pellicle frame in a drumhead-like slack-free fashion. Usually, the other end surface of the pellicle frame, reverse to the end surface to which the pellicle membrane is adhesively bonded, is coated with a pressure-sensitive adhesive so as to ensure reliableness of mounting of the pellicle frame on the photomask.
When a flame-supported pellicle is mounted on the photomask, the dust particles floating in the atmospheric air and falling therefrom are never deposited directly on the photomask but are deposited on the pellicle membrane. Since the light beams used for the pattern-wise exposure to light are focused at the patterned images on the photomask, the dust particles deposited on the pellicle membrane, which is held apart above the photomask by the height of the pellicle frame, have little adverse influences on the quality of the pattern reproduction in the photolithographic patterning work.
Although an assembly of a photomask and a frame-supported pellicle mounted thereon by means of a pressure-sensitive adhesive can be used as such in many times of repeated pattern-wise exposure works as mounted on an exposure machine, it sometimes occurs that the pellicle membrane having a so small thickness is broken in the mounting and demounting procedure of the assembly on and from the exposure machine. Such a trouble takes place more readily after prolonged use of the frame-supported pellicle because exposure to intense ultraviolet light necessarily causes embrittlement of the pellicle membrane due to the photochemical degradation of the polymeric resin forming the membrane. Once the pellicle membrane is broken, needless to say, the broken pellicle must be demounted from the photomask and replaced with a new frame-supported pellicle.
The above mentioned replacement of a broken pellicle with a new pellicle, however, is not a so simple matter as it may seem to be. Since the frame-supported pellicle mounted on a photomask is secured at the position by means of a pressure-sensitive adhesive forming an adhesive layer on one end surface of the pellicle frame or between the end surface of the pellicle frame and the surface of the photomask, the pressure-sensitive adhesive adherent to the photomask surface can never be completely removed when the pellicle frame having the broken pellicle membrane is lifted therefrom. Accordingly, it is necessary for replacing the broken pellicle with a new pellicle that the pressure-sensitive adhesive left adherent to the photomask surface is completely removed before a new pellicle is mounted. The adherent adhesive can of course be removed by a mechanical means, for example, by rubbing with a cloth. Such a mechanical cleaning means, however, can hardly be undertaken because the cleaning work unavoidably involves a risk of damages caused on the expensive photomask if not to mention that complete removal of the adherent adhesive requires a considerable time and labor. Alternatively, the adherent pressure-sensitive adhesive can be dissolved away with an organic solvent or, at least, can be swollen, as a consequence of partial crosslinking of the adhesive molecules by the ultraviolet irradiation, with an organic solvent so as to facilitate mechanical removal thereof from the photomask surface. Use of an organic solvent as a remover agent, however, is undesirable because of the possible adverse influences on the workers' health due to the toxicity thereof in addition to the unpleasant odor.
Accordingly, it is eagerly desired to develop a means to easily demount a flame-supported pellicle from the photomask surface or to develop a frame-supported pellicle easily demountable from the photomask surface without the above described problems and disadvantages in the conventional frame-supported pellicles in the prior art.